Adsorptive separations are employed when it is impractical or impossible to perform a separation using simpler means such as fractional distillation. These separations can be performed using one of more beds of adsorbent operated on a cyclic or swing bed system. However the several benefits provided by continuous operations which provide a uniform product have prompted the development of a process which simulates the continuous countercurrent movement of the adsorbent and process streams. This process technology, known as simulated moving bed (SMB) adsorptive separation, was originally developed as a petroleum refining process to facilitate the removal of straight chain low octane number paraffins from gasoline blending stocks. It, however, found its first large scale commercial application in the separation of longer chain paraffins from kerosene boiling fractions for use in the production of detergent alkylates and in the separation of xylene fractions. Numerous processes are described in the patent literature in which zeolitic adsorbents are used to separate close boiling chemical compounds such as the isomers of dialkyl substituted monocyclic aromatics. For instance the separation of para-xylene from other xylene isomers using simulated moving bed technology is widely described and is now widely practiced commercially in the petrochemicals industry. It is used in the recovery of a large percentage of the para-xylene which is eventually converted into polyester fiber.
More recently it has been discovered that the pharmaceutical effects of various common drugs differs greatly between the different chiral enantiomers of the drug. It has also been found that specific chiral structures are necessary in some drugs now under development for the treatment of a large number of diseases. The separation of enantiomers has therefore become very important in the pharmaceutical industry, and the methods used in other industries to separate compounds having similar physical characteristics are being refined to perform the separation of chiral compounds. Small scale SMB units adapted to the particular needs of the pharmaceutical industry are therefore being developed. There still remains the continuing need to develop the specific stationary phase/mobile phase pairs needed to effect the various desirable separations by adsorption.